1. Field of the Invention
The present invention relates to a fine pattern forming method, and particularly to a method which forms a fine pattern on a substrate by photolithography using near field light.
2. Description of the Prior Art
The evolution of photolithography technology has been supported particularly by the advance of reduced projection exposure technology and resist technology. The performance of reduced projection exposure technology mainly depends upon two basic parameters, i.e., the resolution, RP, and the depth of focus, DOP. If the exposure wavelength for the projection optical system is xcex, and the numerical aperture of the projection lens is NA, the above-mentioned two basic parameters are expressed by RP=k1xcex/NA, and DOP=k2xcex/NA2. In order to improve the resolution for lithography, it is essential to reduce the wavelength xcex, and increase the numerical aperture of the projection lens, NA. However, if NA is increased, the resolution is improved, but the depth of focus is reduced in inverse proportion to the square of NA. Therefore, reduction in wavelength xcex has been demanded in the trend toward finer pattern formation. The exposure wavelength xcex has been shortened from that for the g-line (436 nm) to that for the i-line (365 nm), and at present, the excimer laser (248 nm, 193 nm) has become the most popular.
However, with lithography using light, the diffraction limit for light provides the limit of resolution, and it is generally accepted that, if an F2 excimer laser with a wavelength of 248 nm is used, a fine pattern of 100 nm in line width is the limit of lithography using a lens series optical system. If it is attempted to provide a resolution in the order of less than 100 nm, electron beam or X-ray (particularly SOR light, i.e., synchrotron orbital radiation) lithography technology must be used.
Electron beam lithography can control the formation of a pattern in the order of nanometers with high accuracy, providing a significantly greater depth of focus than that for the optical system. In addition, it offers an advantage that it can directly draw a figure on the wafer without a mask, but because the throughput is low, and the cost is high, it has a drawback that it is far from suited to volume production.
X-ray lithography can provide an approx. one digit higher resolution and accuracy than those for the excimer laser lithography either when full-scale exposure is carried out with a 1-to-1 mask or when a reflection type image formation optical system is used for exposure. However, X-ray lithography presents problems that the mask is difficult to prepare, the feasibility is low, and the cost is high due to the device.
With lithography using an electron beam or X-ray, a resist must be developed in accordance with the exposure method, and problems still exist with respect to sensitivity, resolution, resistance to etching, etc.
As a method for solving the problems described above, a method has been proposed with which near field light effused from openings having a diameter sufficiently smaller than the wavelength of the projected light is used as a light source, and a fine pattern is formed by exposing the resist to the near field light and processing the resist for development. This method allows a spatial resolution in the order of nanometers to be obtained regardless of the wavelength for the light source.
However, unlike the conventional propagated light, the near field light has a propagation depth of as small as several tens of nm (therefore, the word xe2x80x9ceffusedxe2x80x9d is used in place of xe2x80x9cpropagatedxe2x80x9d, and in the drawings, the near field light is depicted as if it were a drop of water hanging from a faucet). It is impossible to expose a thick-film resist having a thickness as large as 1000 nm, and a problem where it is difficult to form a resist pattern with a high aspect ratio is presented. Here, if the line width and the line height for the resist are xe2x80x9caxe2x80x9d and xe2x80x9cbxe2x80x9d, respectively, the aspect ratio is expressed as b/a, and it can be said that, for a given resist thickness, the higher the aspect ratio, the finer the pattern will be.
In addition, there is a problem that, when the substrate has a difference in level, it is difficult to apply the resist to a uniform thickness over the entire surface, even if the thickness of the resist is so small that the near field light can reach the bottom, which means that an area where the light cannot reach the bottom occurs in the resist, and it is extremely difficult to carry out high-precision lithography.
The fine pattern forming method according to the present invention is a fine pattern forming method in which a first resist layer capable of being removed by dry etching, and a photosensitive second resist layer having a resistance to dry etching with which only the irradiated portion or only the non-irradiated portion is made soluble in a developing solvent are stacked together in this order for creating a recording material, and by means for generating near field light on the projected light, the near field light is projected onto the second resist layer of the recording material in the form of a desired pattern. Thereafter, by processing the second resist layer for development, a pattern is formed in the second resist layer, and by using the pattern as a mask, the first resist layer is dry-etched to form a pattern on the substrate of the recording material.
The above-mentioned second resist layer preferably has a film thickness of 100 nm or less.
The fine pattern forming method according to the present invention preferably uses a recording material which provides antireflection means against the projected light on the substrate. In this case, the antireflection means is preferably an antireflection film formed between the substrate and the first resist layer, or an antireflection film formed between the first resist layer and the second resist layer.
The means for generating near field light may be a mask with which the near field light is generated from a metallic pattern formed on a material having a permeability to the projected light, and the metallic pattern is tightly contacted with the second resist layer or brought close thereto within the reach of the near field light for carrying out exposure.
The means for generating near field light may also be an optical stamp with which a convexity and concavity pattern is formed on the surface of a material having a permeability to the projected light, and near field light is generated from the convexity and concavity pattern by total reflection, and the convexity and concavity pattern is tightly contacted with the second resist layer or brought close thereto within the reach of the near field light for carrying out exposure.
The means for generating near field light may also be a probe having an opening with a diameter smaller than the wavelength of the projected light, and the probe is moved on the second resist to carry out exposure.
The second resist layer and the means for generating near field light are preferably tightly contacted with each other by carrying out evacuation in the exposure device for projecting the near field light.
The second resist layer and the means for generating near field light may be tightly contacted with each other by carrying out evacuation in the exposure device and blowing air from the back of the substrate for projecting the near field light.
The first resist layer is preferably etched by oxygen plasma.
The second resist layer preferably comprises a pattern forming material which contains a compound having silicon atoms. The content of the silicon atoms is preferably 1% to 50% of the solid content in the second resist layer.
The second resist layer may comprise a pattern forming material which contains at least one of a naphthoquinone diazide compound and a diazo ketone compound, and a water-insoluble and alkali-soluble silicone-containing polymer.
The second resist layer may comprise a pattern forming material which contains a water-insoluble and alkali-soluble silicone-containing polymer, a compound generating an acid when irradiated with active light beam or radiation, and a high molecular or low molecular compound which has a group capable of being decomposed by an acid, and the solubility of which in an aqueous alkali developing solution is increased under the action of the acid.
The second resist layer may comprise a pattern formation material which contains a water-insoluble silicone-containing polymer which has a group capable of being decomposed by an acid, the solubility of which in an aqueous alkali developing solution is increased under the action of the acid, and which has a functional group, a compound generating an acid when irradiated with active light beam or radiation, and a high molecular or low molecular compound which has a group capable of being crosslinked by the acid.
The second resist layer may comprise a pattern formation material which contains a water-insoluble silicone-containing polymer which has an olefin unsaturated group and the solubility of which in an aqueous alkali developing solution is decreased through the polymerization reaction, and a compound providing a polymerization reaction initiating capability when irradiated with active light beam or radiation.
The second resist layer may comprise a pattern formation material which contains a water-insoluble and alkali-soluble silicone-containing polymer, a compound providing a polymerization reaction initiating capability when irradiated with active light beam or radiation, and a high molecular or low molecular compound which has an olefin unsaturated group and the solubility of which in an aqueous alkali developing solution is decreased through the polymerization reaction.
With the fine pattern forming method of the present invention, a pattern having a line width of 100 nm or less, which is said to be the limit for the conventional photolithography, can be formed by exposing the resist to the near field light effused from the pattern having a line width sufficiently smaller than the wavelength of the projected light, and processing the resist for development.
Up to now, the resolution for lithography has mainly depended upon the wavelength for the light source, but the light source for generating the near field light may have any wavelength, which means there is no need to develop a new light source, which allows a substantial cost reduction.
As the resist, a two-layer resist having a photosensitive resist and a resist of the lower layer comprising organic high molecules is used, which allows the surface to be flattened with the resist of the organic high molecules, and therefore the film thickness of the photosensitive resist of the upper layer to be made uniform, even when the substrate has a difference in level, resulting in an area where the near field light cannot reach the bottom being produced in the first layer. Consequently, the near field light can be uniformly projected even for a pattern having a large area, and a precision pattern of the photosensitive resist can be formed. By using the pattern of the photosensitive resist as a mask for patterning the resist layer of the organic high molecules, which is the lower layer, by the conventional dry-etching method, a fine pattern having a high aspect ratio can be easily formed.
After working the substrate, the two-layer resist can be easily peeled off with a well-known organic solvent which is used as the solvent for the resist, because the organic high molecule resist layer has not been changed by the exposure, which offers an advantage of a good productivity.
Thus, with the fine pattern forming method of the present invention which uses near field light and a two-layer resist, a fine pattern of 100 nm or less can be formed with a high aspect ratio and at a low cost.
The present invention provides a fine pattern forming method which assures a high aspect ratio in lithography using near field light.